The present invention relates to vehicles movable within an ambient fluid, such as air or water. The invention is particularly useful in VTOL (Vertical Take-off and Landing) aircraft, and is therefore described below with respect to this application.
Many different types of VTOL aircraft have been proposed where the weight of the vehicle in hover is carried directly by rotors or propellers, with the axis of rotation perpendicular to the ground. One well known vehicle of this type is the conventional helicopter which includes a large rotor mounted above the vehicle fuselage. Other types of vehicles rely on propellers that are installed inside circular cavities, shrouds, ducts or other types of nacelle, where the propeller or rotor is not exposed, and where the flow of air takes place inside a circular duct. Most ducts have uniform cross-sections with the exit area (usually at the bottom of the duct when the vehicle is hovering) being similar to that of the inlet area (at the top of the duct). Some ducts, however, are slightly divergent, having an exit area that is larger than the inlet area, as this was found to increase efficiency and reduce the power required per unit of lift for a given inlet diameter. Some ducts have a wide inlet lip in order to augment the thrust obtained, especially in hover.
One of the areas where VTOL vehicles are usually more challenging than fixed wing aircraft is the area of stability and control. The main difficulty stems from the fact that, contrary to fixed wing aircraft which accelerate on the ground until enough airspeed is achieved on their flight surfaces, VTOL vehicles hover with sometimes zero forward airspeed. For these vehicles, the control relies on utilizing the rotors or propellers themselves, or the flow of air that they produce, to create control forces and moments around the vehicle""s center of gravity (CG).
One method, which is very common in helicopters, is to mechanically change, by command, the pitch of the rotating rotor blades both collectively and cyclically, and to modify the main thrust as well as moments and/or inclination of the propeller""s thrust line that the propeller or rotor exerts on the vehicle. Some designs use four or more separate propellers, and change the collective angle of all the blades on each propeller. The use of four propellers with collective blade pitch control eliminates the added complexity of changing the angle of each blade individually (termed cyclic control) when using a single rotor for the complete vehicle. On vehicles using multiple propellers/fans which are relatively far from the CG, differential collective control settings can therefore be used on each fan to produce the desired control moments. Another method which can sometimes be used is to rely on fixed pitch propellers, but to change their rotational speed with respect to each other to obtain the increase/decrease of thrust desired from each propeller.
The Curtiss-Wright VZ-7 (VTOL Military Research Aircraft; Mike Rogers, Orion Books N.Y., 1989) is an example of a vehicle in which pitch and roll motions are controlled by changing the blade pitch angle on four free (i.e. unducted) propellers mounted to the sides of a central, elongated fuselage. The Igor Bensen B-12 xe2x80x98SkyMatxe2x80x99 (Janes xe2x80x98All the World""s Aircraftxe2x80x99; Janes publications 1962, England), illustrates an example of a vehicle utilizing a plurality of fixed pitch propellers each mounted to a separate engine. Control moments are generated by increasing and decreasing the RPM on the engines which are mounted on different ends of the vehicle. The propellers are unducted, and the vehicle resembles somewhat the configuration of the VZ-7 but with many more exposed and unducted propellers. In such arrangements, each propeller is exposed and unprotected.
The Paul Moller M200X (Popular Science, March 2000 Times Mirror Magazines), uses a plurality of internally mounted propellers inside ducts. However, each propeller has its own duct which adds to the overall weight of the vehicle; and the area between the propellers is blocked to the air flow which does not enable maximum deck surface of the vehicle to be utilized for lift.
U.S. Pat. No. 6,179,247 describes a VTOL aircraft vehicle in which an annular array of powered thrusters, e.g., rotary propellers, are mounted within an annular duct circumscribing the vehicle with the passenger compartment located centrally of the annular duct. Such an arrangement, among other disadvantages, makes it difficult for the passenger or passengers to enter and exit
An object of the present invention is to provide a vehicle generally, and particularly a VTOL aircraft which, having advantages in one or more of the above respects.
According to one aspect of the present invention, there is provided a vehicle movable within an ambient fluid, comprising: a vehicle body; a first group of rotary propeller propulsion units mounted on one side of the vehicle body, each of the rotary propeller propulsion units being oriented to produce a downward flow of the fluid and thereby an upward lift force applied to that side of the vehicle body; a second group of rotary propeller propulsion units mounted on the opposite side of the vehicle body, each of the latter rotary propeller propulsion units being oriented to produce a downward flow of the fluid, and thereby an upward lift force applied to the opposite side of the vehicle body; a first elongated duct on the one side of the vehicle body enclosing all the rotary propeller propulsion units of the first group and having an upper end and a lower end projecting past the upper and lower ends of the enclosed rotary propeller propulsion units; and a second elongated duct, separate from the first elongated duct, located on the opposite side of the vehicle body and enclosing all the rotary propeller propulsion units of the second group, the second elongated duct having an upper end and a lower end projecting past the upper and lower ends of the second group of rotary propeller propulsion units; the cross-sectional area of each of the first and second elongated ducts being larger than the sum of the cross-sectional areas of the plurality of rotary propeller propulsion units enclosed by the respective elongated duct, to increase the efficiency whereby the lift force is produced by the respective rotary propeller propulsion units; the ends of the first and second elongated ducts being spaced from each other to facilitate entry and exit from the vehicle.
According to further features in the preferred embodiments of the invention described below, the upper inlet end of each elongated duct is circumscribed by a curved surface which guides smoothly the fluid flow into the duct and also further augment the lift force produced by the propulsion units, through the Bernoulli effect.
As will be described more particularly below, the invention is implemented in a VTOL aircraft in which the propulsion units are rotary propellers. While using a plurality of propellers within a single duct will have some detrimental effect on the thrust augmentation achieved by the duct because of the inability to completely avoid gaps between propellers and the duct walls, such an arrangement still provides some augmentation, while keeping the propellers protected inside the fuselage, while at the same time enabling a simple control scheme to be used for controlling the vehicle in flight. It is also believed that, although some regions inside the duct have no propellers directly acting on the air stream, the overall flow through the duct will be relatively uniform as long as the propellers are not separated too far from each other.
It is believed, therefore, that a vehicle constructed in accordance with the foregoing features, particularly a VTOL aircraft including propellers as the propulsion units, will provide a number of important advantages including the following: since the propellers are inside ducts and within the confines of the fuselage, the arrangement provides protection to the propeller blades, as well thrust augmentation by the duct. In addition, the total duct cross-sectional area is larger than the sum of the separate cross-sectional areas of each propeller. Further, the thrust of each propeller can be changed, so that a simple scheme for creating control movements can be obtained.
Further features and advantages of the invention will be apparent from the description below.